core: Fix return argument check for parse_emergency_action

core: Fix return argument check for parse_emergency_action

This function returns 0 on success and a negative value on failure. On success,
it writes the parsed action to the address passed in its third argument.

`bus_set_transient_emergency_action` does this:

 r = parse_emergency_action(s, system, &v);
 if (v < 0)
     // handle failure

However, `v` is not updated if the function fails, and this should be checking
`r` instead of `v`.

The result of this is that if an invalid failure (or success) action is
specified, systemd ends up creating the unit anyway and then misbehaves if it
tries to run the failure action because the action value comes from
uninitialized stack data. In my case, this resulted in a failed assertion:

 Program received signal SIGABRT, Aborted.
 0x00007fe52cca0d7f in raise () from /snap/usr/lib/libc.so.6
 (gdb) bt
 #0  0x00007fe52cca0d7f in raise () from /snap/usr/lib/libc.so.6
 #1  0x00007fe52cc8b672 in abort () from /snap/usr/lib/libc.so.6
 #2  0x00007fe52d66f169 in log_assert_failed_realm (realm=LOG_REALM_SYSTEMD, text=0x56177ab8e000 "action < _EMERGENCY_ACTION_MAX", file=0x56177ab8dfb8 "../src/core/emergency-action.c", line=33, func=0x56177ab8e2b0 <__PRETTY_FUNCTION__.14207> "emergency_action") at ../src/basic/log.c:795
 #3  0x000056177aa98cf4 in emergency_action (m=0x56177c992cb0, action=2059118610, options=(unknown: 0), reboot_arg=0x0, exit_status=1, reason=0x7ffdd2df4290 "unit run-u0.service failed") at ../src/core/emergency-action.c:33
 #4  0x000056177ab2b739 in unit_notify (u=0x56177c9eb340, os=UNIT_ACTIVE, ns=UNIT_FAILED, flags=(unknown: 0)) at ../src/core/unit.c:2504
 #5  0x000056177aaf62ed in service_set_state (s=0x56177c9eb340, state=SERVICE_FAILED) at ../src/core/service.c:1104
 #6  0x000056177aaf8a29 in service_enter_dead (s=0x56177c9eb340, f=SERVICE_SUCCESS, allow_restart=true) at ../src/core/service.c:1712
 #7  0x000056177aaf9233 in service_enter_signal (s=0x56177c9eb340, state=SERVICE_FINAL_SIGKILL, f=SERVICE_SUCCESS) at ../src/core/service.c:1854
 #8  0x000056177aaf921b in service_enter_signal (s=0x56177c9eb340, state=SERVICE_FINAL_SIGTERM, f=SERVICE_SUCCESS) at ../src/core/service.c:1852
 #9  0x000056177aaf8eb3 in service_enter_stop_post (s=0x56177c9eb340, f=SERVICE_SUCCESS) at ../src/core/service.c:1788
 #10 0x000056177aaf91eb in service_enter_signal (s=0x56177c9eb340, state=SERVICE_STOP_SIGKILL, f=SERVICE_SUCCESS) at ../src/core/service.c:1850
 #11 0x000056177aaf91bc in service_enter_signal (s=0x56177c9eb340, state=SERVICE_STOP_SIGTERM, f=SERVICE_FAILURE_EXIT_CODE) at ../src/core/service.c:1848
 #12 0x000056177aaf9759 in service_enter_running (s=0x56177c9eb340, f=SERVICE_FAILURE_EXIT_CODE) at ../src/core/service.c:1941
 #13 0x000056177ab005b7 in service_sigchld_event (u=0x56177c9eb340, pid=112, code=1, status=1) at ../src/core/service.c:3296
 #14 0x000056177aad84b5 in manager_invoke_sigchld_event (m=0x56177c992cb0, u=0x56177c9eb340, si=0x7ffdd2df48f0) at ../src/core/manager.c:2444
 #15 0x000056177aad88df in manager_dispatch_sigchld (source=0x56177c994710, userdata=0x56177c992cb0) at ../src/core/manager.c:2508
 #16 0x00007fe52d72f807 in source_dispatch (s=0x56177c994710) at ../src/libsystemd/sd-event/sd-event.c:2846
 #17 0x00007fe52d730f7d in sd_event_dispatch (e=0x56177c993530) at ../src/libsystemd/sd-event/sd-event.c:3229
 #18 0x00007fe52d73142e in sd_event_run (e=0x56177c993530, timeout=18446744073709551615) at ../src/libsystemd/sd-event/sd-event.c:3286
 #19 0x000056177aad9f71 in manager_loop (m=0x56177c992cb0) at ../src/core/manager.c:2906
 #20 0x000056177aa7c876 in invoke_main_loop (m=0x56177c992cb0, ret_reexecute=0x7ffdd2df4bff, ret_retval=0x7ffdd2df4c04, ret_shutdown_verb=0x7ffdd2df4c58, ret_fds=0x7ffdd2df4c70, ret_switch_root_dir=0x7ffdd2df4c48, ret_switch_root_init=0x7ffdd2df4c50, ret_error_message=0x7ffdd2df4c60) at ../src/core/main.c:1792
 #21 0x000056177aa7f251 in main (argc=2, argv=0x7ffdd2df4e78) at ../src/core/main.c:2573

Fix this by checking the correct variable.